Other question: would it really come to mind to someone who wants to hide to shut down the ACARS? To shut that down first and then ten minutes later, the transponder? The opposite sequence would make so much more sense. Or both roughly at the same time.
Perhaps it was done surreptitiously, being an uncommon action, the other pilot did not noticed or questioned. Only after few minutes, having incapacitated or killed the other pilot, xponder was shut down and the rest of the actions took place.